Ultraviolet curable basecoats for vacuum metallization

ABSTRACT

An ultraviolet light-curable liquid coating composition for molded plastic objects is disclosed. The composition is a liquid mixture of a polyurethane oligomer polyacrylate and an hydroxy-functional polyacrylate ester. The polyacrylate ester is present in an amount of 5% to 95% of total polyacrylate. The composition is preferably in solution in alcoholic solvent having a solids content of from 40% to 90%. A photoinitiator is included to render the composition curable upon exposure to ultraviolet light.

DESCRIPTION

1. Field of Invention

This invention relates to ultraviolet light-curable basecoats forthermoplastic molded objects to prepare them for vacuum metallization.

2. Background Art

Thermoplastic molded objects, particularly those intended for use on theexterior of automobiles, are decorated by vacuum metallization toprovide a bright and shiny metallic appearance. However, these moldedparts frequently lack the smooth surface needed to obtain the desiredbright and shiny finish, so the molded parts are coated to improvesurface smoothness, and the coating is baked to cure the same. Baking atlow temperature not exceeding about 170° F. is required because themolded plastic parts are damaged by exposure to higher temperature.However, a baking time of about 2 hours is needed at 170° F., and thisholds up production and introduces large oven requirements and energycosts which are detrimental. Moreover, the prior basecoats exhibit poorhumidity and salt spray resistance evidenced by delamination of themetal deposit.

The use of ultraviolet light-cured coatings is known to eliminate theneed for an extended bake, but ultraviolet-cured coatings normallyinclude residual volatile material, and thus would not be regarded ascapable of withstanding a vacuum metallization process because the lossof volatiles would disrupt the coating and impair the desired bright andshiny finish. Thus, while light-cured coatings constitute a known way ofeliminating prolonged baking, such coatings would not normally beregarded to be applicable to the problem under consideration.

DISCLOSURE OF INVENTION

In accordance with this invention, an ultraviolet light-curable lightcoating composition for molded plastic objects comprises a liquidmixture of: (1) a polyurethane oligomer polyacrylate; and (2) anhydroxyfunctional polyacrylate ester; the polyacrylate ester beingpresent in an amount of from 5% to 95%, more preferably from 20% to 50%of total polyacrylate. Ultraviolet curing systems normally contain novolatile organic solvent, but the preferred coating compositions of thisinvention contain alcoholic organic solvent providing a solids contentof 40% to 90%, preferably from 60% to 80%. A photoinititor is added torender the coating composition curable upon exposure to ultravioletlight (which includes visible light near the ultraviolet range).

All proportions are by weight unless otherwise noted.

The resulting solution is dip or spray applied and the wet-coatedarticle is exposed to ultraviolet light which provides a rapid cure,usually in 5 to 60 seconds.

Photoinitiator selection is subject to wide variation, as is well knownin the ultraviolet cure of acrylate-containing liquids. Ketonicphotoinitiators are preferred, such as those of the acetophenone type.The selection of photoinitiators is not a critical aspect of thisinvention. The acetophenone photoinitiator provided by Ciba-GeigyCorporation under the name Irgacure 651 is preferred and will be used insome of the examples. From 2-10%, preferably from 5-8% photoinitiator isusually appropriate.

The polyurethane oligomer polyacrylates are well known materials whichare usually prepared by reacting a low molecular weight, organicsolvent-soluble, hydroxy functional polyester with excess organicpolyisocyanate (usually a diisocyanate like toluene diisocyanate) toform an isocyanate-terminated polyurethane which is then reacted with anhydroxy alkyl acrylate, such as 2-hydroxyethyl acrylate, to consume theisocyanate functionality and thus provide a plurality of terminalunsaturated acrylate groups. Similar products can be made in variousways and well known in the art.

Preferred polyurethane oligomer polyacrylates in this invention arebased on polyesters having an average molecular weight of from 500 to1500 and having more than about 1.5 hydroxy groups per molecule,preferably at least about 2.0 hydroxy groups per molecule. Each hydroxygroup in the polyester is bonded to an acrylate group through aplurality of urethane linkages to provide the desired polyacrylatepolyurethane oligomers under consideration.

The hydroxy-functional polyacrylate ester can be constituted by anorganic polyol having a molecular weight below 500 and having at leastabout 3 hydroxy groups per molecule which is esterified with at leastabout 1.5 mols of acrylic acid per mol of polyol, preferably at leastabout 2.0 mols of acrylic acid on the same basis. The esterificationgoes to substantial completion to remove most of the unreacted acrylicacid, and at least about 0.5 hydroxy groups per molecule are leftunreacted. These polyacrylate esters are illustrated by trimethylolpropane diacrylate and pentaerythritol triacrylate. The residual hydroxyfunctionality promotes adhesion to both plastic and metal, so thesubstitution of trimethylol propane triacrylate for the correspondingdiacrylate is not satisfactory. Glycerin diacrylate can also be used toillustrate a polyacrylate ester of relatively low molecular weight, anda diacrylate of diglycidyl ether of bisphenol A having a molecular-weight of about 375 illustrates a polyacrylate of higher molecularweight. These higher molecular weight polyacrylates can be provided bythe reaction of a polyepoxide with acrylic acid to produce alpha hydroxyesters.

In the preferred solvent-containing compositions, the use of alcohols isnormally needed to avoid attacking the plastic which is coated. Smallamounts of another solvent, up to about 25% of total solvent, may beconstituted a more active solvent, like butyl acetate or methyl ethylketone, but the system should remain largely alcoholic to minimizesolvent attack of the substrate. When water miscible alcohols areemployed, like ethanol, some water may be tolerated, though thisincreases the dry time needed prior to cure.

The coating composititons which are contemplated may be applied invarious ways, as by spray or by dipping. Dipping is particularlypreferred herein.

The molded objects are dipped into a bath of the coating solution andexcess solution is drained off, usually back into the bath. Adequatedrainage usually takes 1 to 2 minutes. The object is then dried toremove the solvent which is preferably present and thereby provide atacky uniform film. This takes about 5 to 15 minutes in air, or the drycan be speeded by blowing warm air on the object. In this way one cancarry out the required drying in about 3 to 5 minutes.

It will be appreciated that the dip procedure automatically controls thecoating thickness and uniformity and it avoids the spray losses whichare encountered when some of the spray particles do not deposit upon theobject to be coated.

The tacky film is a solid film, but touching with the fingers will leavean observable print thereon.

The films which are formed on air drying to remove the bulk of thevolatile solvents which are preferably present are then exposed toultraviolet light causing a rapid cure. Typical cure times are 10 to 15seconds using an ultraviolet source supplying an intensity of 1.0 to 3.0Joules/cm².

The cured films obtained by this invention are very uniform. The edgecoverage is good, and the coatings flow well to provide a smooth surfacewhich allows the subsequently applied metal film to be bright and shiny.

The vacuum metallization process is itself well-known, the process beingnormally carried out using a metal or metal alloy, such as aluminum orchromium. The vacuum metallization process is well known and encompassesevaporative as well as sputtering techniques.

The invention is illustrated in the examples which follow:

EXAMPLE 1

    ______________________________________                                        Component             Parts by Weight                                         ______________________________________                                        Acrylated polyurethane                                                                              27                                                      oligomer (see note 1)                                                         Pentaerythritol triacrylate                                                                         79                                                      Dimethoxy phenyl acetophenone (note 2)                                                               8                                                      Methanol              25                                                      Isopropanol           12                                                      Butanol               12                                                      Total =               163                                                     ______________________________________                                    

The product has a solids content of 70% and a #2 Zahn viscosity of 20seconds at room temperature.

Note 1--A polyester of adipic acid and propylene glycol of molecularweight 1000 and an hydroxyl equivalent weight of 500 reacted with excessaliphatic diisocyanate and then with hydroxyethyl acrylate (Thiokolproduct Uvithane 788 may be used).

Note 2--Ciba Geigy product Iragacure 651 may be used.

EXAMPLE 2

    ______________________________________                                        Component           Parts by Weight                                           ______________________________________                                        Acrylated polyurethane                                                                            30                                                        oligomer (see note 1)                                                         Bisphenol A epoxy diacrylate                                                                      30                                                        (see note 3)                                                                  Pentaerythritol triacrylate                                                                       40                                                        Benzophenone         3                                                        Diethoxy acetophenone                                                                              3                                                        Methanol            37                                                        Isopropyl alcohol   10                                                        Butyl acetate       10                                                        Total =             163                                                       ______________________________________                                    

Note 3--The commercial product Celrad 3700 from Celanese may be used.

The product has a solids content of 65% and a #2 Zahn viscosity of 23seconds at room temperature.

EXAMPLE 3

    ______________________________________                                        Component             Parts by Weight                                         ______________________________________                                        Acrylated polyurethane                                                                              53                                                      oligomer (see note 1)                                                         Pentaerythritol triacrylate                                                                         53                                                      Benzophenone           4                                                      Dimethoxy phenyl acetophenone (note 2)                                                               4                                                      Methanol              30                                                      Isopropanol           19                                                      Butanol               12                                                      Total =               175                                                     ______________________________________                                    

The product has a solids content of 65% and a #2 Zahn viscosity of 20seconds at room temperature.

EXAMPLE 4

    ______________________________________                                        Component             Parts by Weight                                         ______________________________________                                        Acrylated polyurethane                                                                              53                                                      oligomer (see note 1)                                                         Pentaerythritol triacrylate                                                                         53                                                      Dimethoxy phenyl acetophenone (note 2)                                                               8                                                      Methanol              30                                                      Isopropanol           15                                                      Butanol                8                                                      Butyl acetate          8                                                      ______________________________________                                    

The product has a solids content of 65% and a #2 Zahn viscosity of 20seconds. It is appropriate for dip application, draining, drying andultraviolet cure as previously described.

EXAMPLE 5

    ______________________________________                                        Component         Parts by Weight                                             ______________________________________                                        Acrylated polyurethane                                                                          85                                                          oligomer (see note 4)                                                         Pentaerythritol triacrylate                                                                      5                                                          Phenoxy ethyl acrylate                                                                          10                                                          Benzophenone       2                                                          Diethoxyacetophenone                                                                             4                                                          Isopropanol       47                                                          Butanol           24                                                          Total =           177                                                         ______________________________________                                    

Note 4--The commercial product Uvimer 530 of Polychrome Corp. may beused.

This Example 5 product has a solids content of 60% and a #2 Zahnviscosity of 20 seconds at room temperature.

EXAMPLE 6

    ______________________________________                                        Component             Parts by Weight                                         ______________________________________                                        Acrylated polyurethane                                                                              70                                                      oligomer (see note 4)                                                         Bisphenol A epoxy diacrylate                                                                        15                                                      (see note 5)                                                                  Pentaerythritol triacrylate                                                                         15                                                      Dimethoxy phenyl acetophenone (note 2)                                                               6                                                      Isopropanol           41                                                      Butanol               20                                                      Butyl acetate         10                                                      Total =               177                                                     ______________________________________                                    

Note 5--The commercial product DRH 370 from Shell Chemical may be used.

The product of this example has a solids content of 60% and a #2 Zahnviscosity of 22 seconds at room temperature.

EXAMPLE 7

    ______________________________________                                        Component             Parts by Weight                                         ______________________________________                                        Acrylated polyurethane                                                                              15                                                      Oligomer (see note 4)                                                         Bisphenol A epoxy diacrylate                                                                        75                                                      (see note 5)                                                                  Phenoxy ethyl acrylate                                                                              10                                                      Dimethoxy phenyl acetophenone (note 2)                                                               6                                                      Isopropanol           25                                                      Butyl acetate         10                                                      Total =               141                                                     ______________________________________                                    

This product has a solids content of 75% and a #2 Zahn viscosity of 23seconds at room temperature.

EXAMPLE 8

    ______________________________________                                        Component           Parts by Weight                                           ______________________________________                                        Acrylated polyurethane                                                                            186                                                       oligomer (see note 6)                                                         Pentaerythritol triacrylate                                                                        14                                                       Dimethoxy phenyl acetophenone                                                                      16                                                       (see note 2)                                                                  Isopropanol         122                                                       Butanol              23                                                       Total =             361                                                       ______________________________________                                    

Note 6--A polyester of trimethylol propane and adipic acid having amolecular weight of 800 and an hydroxyl equivalent weight of about 266reacted with excess aliphatic diisocyanate and then with hydroxyethylacrylate. The commercial product Uvimer 775 of Polychrome Corp. ofNewark, New Jersey may be used.

The aliphatic diisocyanates referred to hereinbefore are constituted bydicyclohexyl methane 4,4'-diisocyanate. Hylene W supplied by Mobay maybe used.

This Example 8 has a solids content of 55% and a #2 Zahn viscosity of 25seconds. It is also appropriate for dip application, draining, dryingand ultraviolet cure as previously described.

EXAMPLE 9

    ______________________________________                                        Component           Parts by Weight                                           ______________________________________                                        Acrylated polyurethane                                                                            10                                                        oligomer (see note 6)                                                         Pentaerythritol polyacrylate                                                                      80                                                        (see note 7)                                                                  Phenoxy ethyl acrylate                                                                            10                                                        Dimethoxy phenyl acetophenone                                                 (see note 2)         6                                                        Total =             106                                                       ______________________________________                                    

Note 7--The commercial product ZA1192 from Thiokol may be used.

The product of this Example 9 has a solids content of 100% and a #2 Zahnviscosity of 65 seconds at room temperature and 26 seconds at 100° F.

As will be evident, small amounts of extraneous materials, such asmonoacrylates like 2-hydroxyethyl acrylate, or polyacrylates likediethylene glycol diacrylate, soluble or insoluble resin particles,fillers, flow control agents, and the like may be present. Also, andwhile it is preferred to apply the liquid composition at roomtemperature, heat can be used to reduce the viscosity or to assist inthe elimination of all or a portion of the volatile organic solvent.

What is claimed is:
 1. An ultraviolet light-curable liquid coatingcomposition for molded plastic objects comprising an alcoholic solventsolution of:1--a polyurethane oligomer polyacrylate which is aderivative of an organic solvent-soluble hydroxy functional polyesterhaving an average molecular weight of 500-1500 and an average of atleast about 2.0 hydroxy groups per molecule, each hydroxy group carryingan acrylate ester group by a plurality of urethane linkages; and 2--anhydroxy-functional polyacrylate which is constituted by an organicpolyol having a molecular weight below 500 and at least about 3 hydroxygroups per molecule, said polyol being esterified to contain at leastabout 1.5 acrylate ester groups per molecule and having at least about0.5 unreacted hydroxy groups per molecule;said polyacrylate ester beingpresent in an amount of from 20-50% of total polyacrylate, and saidsolution having a solids content of from 60% to 80%, said solutionfurther including a photoinitiator to render the composition curable onexposure to ultraviolet light.